1. Field of the Invention
This invention relates to a method of imaging a source of radiation and to a device for imaging a source of radiation, and more specifically, this invention relates to a method and device for producing a three-dimensional image of a source of x-ray and gamma-ray radiation for medical and other applications by using a plurality of diffracting crystals which focus x-ray and gamma-ray radiation onto a plurality of detection devices.
2. Background of the Invention
Cancer tumor cells have high rates of metabolism and multiply rapidly. Substances injected into the body tend to migrate to locations of such high growth and become incorporated in this new growth. If the injected substance is a short-lived radioactive isotope, the location of a tumor can be detected by locating the region of high radioactivity. Aside from pinpointing tumor location, an image of the tumor is also desirable to ascertain its shape, size, and juxtaposition with adjacent structures.
One method used to detect tumors is to first inject a body with radioactive compounds such as the Technetium isotope .sup.99m Tc, which is a 140.5 kiloelectron volt (keV) gamma emitter having a half-life of 5.9 hours. The gamma rays are detected by a large sodium iodide (Nal) scintillator crystal placed behind a collimator grid yielding at best an 8 millimeter (mm) resolution at the location of the source. The scintillator is viewed by a plurality of photomultiplier tubes and the location of a scintillation event is determined by a computer analysis of the relative intensity of the photomultiplier signals. The collimator/scintillator assembly is placed above and very close to the patient. Aside from this method yielding a low resolution of between approximately 8 mm and 1 centimeter (cm), the image produced is limited to the plane parallel to the surface of the scintillator. As such, the technique provides no depth information about the source. This deficiency can be remedied somewhat by adding another collimator/scintillator assembly below the patient, comparing the counting rate of the two scintillators, and thus estimating the position of the source along the line joining them. In the latest revision of this method the large Na I detector plus collimator is rotated around the patient, taking a plurality of images at different angles. This allows one to generate a three-dimensional image of the radiation emitting area. There are considerable additional costs associated with this method and the fact that this method has been introduced in spite of the additional costs underscores the importance of three-dimensional imaging.
Another popular imaging technique is positron emission tomography (PET), used in diagnosis and medical research. In PET, a chemical compound containing a short-lived, positron-emitting radioisotope is injected into the body. The positrons (positively charged beta particles) are emitted as the isotope decays. These particles annihilate with electrons in surrounding tissue. Each annihilation simultaneously produces two 511 (keV) gamma rays traveling in opposite directions. After passing through collimators, these two gamma rays are detected simultaneously by scintillation detectors placed at 180 degrees to each other, and on opposite sides of the patient. The signals from the detectors' photomultiplier tubes are analyzed by a computer to facilitate the production of an image of the radiation-emitting region.
Numerous drawbacks exist with scintillation detector tomography. For instance, the typical coarse resolution of no less than 8 mm often results in smaller structures being overlooked. This prevents early detection of cancerous tumors when they are least likely to have metastasized and when treatment is most likely to succeed. This is especially a disadvantage in the detection of breast cancer tumors wherein the tumors often become virulent before growing to a detectable size. Presently, mammography uses x-rays to detect tissue calcification. The assumption is made that this calcification is due to dead cancer cells and that there is a live cancer tumor in the immediate vicinity. Often however, there is no live tumor where calcification has been detected. In fact, the calcification may not have been due to a tumor at all. Unfortunately then, positive mammography results often lead to unnecessary surgical operations.
Also, because poor spacial resolution often causes images of actual small tumors to be diffuse, variations in background radiation are often mistaken for actual tumors, leading to unnecessary surgical operations. This inadvertent incorporation of background radiation is an artifact of scintillation detector use wherein the detector must be large enough to cover a given area of the body. Aside from intercepting the radiation emanating from the source under observation, however, the large detectors also detect all ambient background radiation penetrating the scintillating region and this ambient radiation is analyzed as if it had been emitted by the source under observation.
Another drawback to using imaging techniques incorporating scintillation detectors is that all of the various radiations emitted by the source are detected by the detectors. As such, a specific radiation having an energy indicative of a specific, injected isotope cannot be easily scrutinized.
Lastly, because collimators allow for the detection of only the radiation that is emitted in a very narrow direction in space, the patient has to be injected with a relatively large amount of radioactive material.
Recently, efforts have been made to improve scintillation detector tomography. Some PET instruments now achieve a resolution as small as 4 mm. Such improvements entail considerable expenditures and have the additional drawback that the improvement in resolution has come at the cost of a decrease in counting rate. This entails in turn either a longer examination time per patient or the injection of a stronger dose of radiation. Furthermore, the prospects for further improvements in resolution are limited by the fact that such improvements require collimators with ever smaller apertures, and therefore greater mass, together with lower count rates. This increase in collimator mass will increase the number of forward Compton-scattered photons in the collimators and these forward scattered photons are often indistinguishable from those emanating directly from the source.
To obtain significant improvements in spacial resolution and in detection efficiency as well as a three dimensional location of the source, a method for focusing the radiation emanating from the source is required. Laboratory instruments utilizing the phenomenon of crystal diffraction have been used to focus x-ray and gamma-ray radiation. For instance, U.S. Pat. No. 4,429,411, issued Jan. 31, 1984, to the applicant, discloses a method of focussing x-rays and gamma rays by using bent crystals, crystals that are differentially heated, and crystals that have been differentially doped with impurities. Because of the limits on the amounts of crystal bending, differential heating, or differential doping that can be achieved in practice, these methods do not allow for devices with a focal length of less than a few meters. Aside from the problems associated with situating such large devices in a medical facility and manipulating the associated large componentry thereof, the long focal length of such large devices would also result in low detection efficiencies. Therefore, the imaging patient would need to be injected with a relatively large amount of radiation.
A need exists in the art for a method and device for imaging x-ray and gamma-ray sources with sufficient spacial resolution to accurately observe structures smaller than 8 mm in size. The method and device must have sufficient energy resolution to allow the imaging of radiation of a selected energy to the exclusion of others. The method and device also must be embodied in a manageable size for easy manipulation of pertinent portions of the method or device so as to customize imaging sessions to multiple, specific radiation energies. The method and device also must limit the radiation to which the patient is exposed by incorporating a redirecting or "focussing" mechanism to detect radiation emanating from a tumor while disregarding ambient levels of radiation.